The present invention relates to a semiconductor device and a method of forming a contact plug of a semiconductor device. According to the method, a first dielectric layer is formed on a semiconductor substrate in which junction regions are formed. A hard mask is formed on the first dielectric layer. The hard mask and the first dielectric layer corresponding to the junction regions are etched to form trenches. Spacers are formed on sidewalls of the trenches. Contact holes are formed in the first dielectric layer using an etch process employing the spacers and the hard mask so that the junction regions are exposed. The contact holes are gap filled with a conductive material, thus forming contact plugs. Accordingly, bit lines can be easily formed on the contact plugs formed at narrow spaces with a high density.